The present invention relates generally to an x-ray tube target assembly, and, more particularly to a graphite target assembly with improved mechanical joints.
X-ray tubes are well known and widely utilized in a variety of medical imaging fields, medical therapy fields, and material testing and analysis industries. They are commonly comprised of both an anode assembly and a cathode assembly. X-rays are produced when electrons are released in a vacuum with the tube, accelerated and then abruptly stopped. The electrons are released from a heated filament. A high voltage between the anode and the accelerates the electrons and causes them to impinge on the anode. The anode is also referred to as the target since the electrons impact the anode at the focal spot.
In order to dissipate the heat generated at the focal spot, X-ray tubes often incorporate a rotating anode structure. The anode in these arrangements commonly comprises a rotating disc so that the electron beam constantly strikes a different point on the target surface. Although these methods can reduce the concentration of heat at a single spot on the target surface, there is still considerable heat generated within the target. The rotating disc and rotating shaft assembly may, therefore, be exposed to high temperatures in addition to significant temperature fluctuations between operational states. These temperature fluctuations can expose the components of a target assembly as well as their attachment means to considerable expansion induced stresses.
Such is often the case with graphite and graphite composite target assemblies. The joints between the elements of the target assemblies are often exposed to significant tension loading during cooling after operation. This can cause the fracture or weakening of joint assemblies. This joint stress phenomenon can be even further exacerbated by the use of materials, such as the mentioned graphite composites, with differing coefficients of thermal expansion (CTE). When these materials with CTE mismatches are joined, the heating and cooling phases of the target assembly can induce significant stresses on the joints. Many existing arrangements are forced to rely solely on mechanical joints in order to avoid joint destruction as a result of these stresses. Mechanical joints, however, must be formed with tight tolerances and their associated costs, can require complex machining operations, and are themselves susceptible to stresses resulting from differing cooling/heating rates. Alternatives to conventional welding processes, such as inertia welding (I-welding), are often required as welding cannot often be utilized in cases of large CTE mismatch, the use of graphite materials, non-weldable material combinations, brittle or extremely hard materials, and where significant work has made the material unweldable.
Thus, present target assemblies often do not provide attachment methodologies suitable for exposure to the CTE mismatch or the temperature fluctuations normally experienced by x-ray tube target assemblies. Furthermore, present target assemblies often require overly expensive tooling and manufacturing methodologies. It would, therefore, be highly desirable to have a target assembly with an improved design such that robust mechanical joints were present between target disc members and target shaft members.